Magnetic Resonance Imaging (MRI) can generate cross-sectional images in any plane (including oblique planes). Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water and fat. When the object to be imaged is placed in a powerful, uniform magnetic field the spins of the atomic nuclei with non-integer spin numbers within the tissue all align either parallel to the magnetic field or anti-parallel. The output result of an MRI scan is an MRI contrast image or a series of MRI contrast images.
In order to understand MRI contrast, it is important to have some understanding of the time constants involved in relaxation processes that establish equilibrium following RF excitation. As the high-energy nuclei relax and realign, they emit energy at rates which are recorded to provide information about their environment. The realignment of nuclear spins with the magnetic field is termed longitudinal relaxation and the time (typically about 1 sec) required for a certain percentage of the tissue nuclei to realign is termed “Time 1” or T1. T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time (typically <100 ms for tissue) is termed “Time 2” or T2. On the scanner console all available parameters, such as echo time TE, repetition time TR, flip angle α and the application of preparation pulses (and many more), are set to a certain value. Each specific set of parameters generates a particular signal intensity in the resulting images depending on the characteristics of the measured tissue.
Image contrast is then created by using a selection of image acquisition parameters that weights signal by T1, T2 or no relaxation time PD (“proton-density images”). Both T1-weighted and T2-weighted images as well as PD images are acquired for most medical examinations. The RF excitation of the MR scanner is performed by a rotating B1 field inside an RF transmission coil. This coil is designed to generate a homogeneous B1 field such that the RF excitation is identical throughout the imaged object. In practice imperfection of the coil design and the presence of the imaged object itself may distort the B1 field such that the RF excitation, and with that the flip angle α, may deviate from the intended value. This has an unanticipated effect on image contrast.
In conventional contrast imaging the absolute signal intensity observed in the images has no direct meaning; it is rather the intensity difference, the contrast, between different tissues that lead to a diagnosis. A more quantitative approach can be applied based on the measured physical parameters T1, T2 and PD (the B1 field is merely used to correct for scanner imperfections). Using T1, T2 and PD MR images can be synthesized that are very similar to conventional MR images but with a free choice of scanner setting TE, TR, α and pre-pulses. Moreover T1, T2 and PD form a robust input for tissue segmentation and classification which could lead to MR computer aided diagnose.
There is a constant need to improve diagnostic and imaging methods relating to MRI.